1. Field of the Invention
The present invention relates generally to an apparatus and method for character recognition, and, more particularly, to an apparatus and method for recognizing characters by calculating feature values according to the orientation of each of the characters and classifying the calculated feature values by character type.
2. Description of the Related Art
General character recognition is widely used in fields such as optical character recognition, automatic mail sorting, document recognition, and drawing recognition, as a field of pattern recognition for recognizing and understanding characters by using vision information.
Such a character recognition method includes a character recognition method using a mesh feature, a character recognition method using a directional angle feature, a character recognition method using a stroke orientation feature, a method using the size of a Kirsch mask, a grapheme-based Korean character recognition method, and a character recognition method according to 6 forms of Hangul (Korean) characters.
The character recognition method using a mesh feature is a method of dividing an area to be recognized into several sub-areas and using a density value of black colored pixels existing in each sub-area as a feature.
The character recognition method using a directional angle feature is a method of extracting a direction vector based on distribution of pixels and using the extracted direction vector as a feature. Since direction vectors are extracted for all directions of pixels in a mask window, various pixel patterns can be considered.
The character recognition method using a stroke orientation feature is a method of calculating percentages of length values of a horizontal stroke, a vertical stroke, a left 45-degree diagonal line, and a right 45-degree diagonal line for black colored pixels and using the percentages as a feature.
The method using the size of a Kirsch mask is a method of extracting 3×3 Kirsch masks for a horizontal angle, a vertical angle a right 45-degree angle, and a left 45-degree angle and using size information of the Kirsch masks as a feature.
The grapheme-based Korean character recognition method is classified into a method of extracting characters by using the above-described features, recognizing the characters based on a feature value on a grapheme basis, and combining a result of the recognition and a method of recognizing characters in which graphemes are combined.
The character recognition method according to the 6 forms of Hangul is a method of classifying combinations of an initial consonant and a vertical vowel, combinations of an initial consonant and a horizontal vowel, combinations of an initial consonant, a horizontal vowel and a vertical vowel, initial consonants, vertical vowels, and the like by using a type classification neural network according to the 6 forms of Hangul and forming a recognizer with a neural network or a Hidden Markov Model (HMM) suitable for each form.
As described above, characters are conventionally recognized by using the character recognition method described above.
However, the character recognition method using a mesh feature has a low recognition ratio since it is difficult to distinguish similar Korean fonts, such as “◯” and “□”, from each other, and the character recognition method using a directional angle feature has a low recognition ratio for characters, such as Korean characters, requiring a weight on a diagonal stroke since the same weight is used for every direction in a process of performing grouping with arbitrary angles and setting weights for features.
Additionally, since the character recognition method using a stroke orientation feature is based on a stroke length, when strokes are cut or exist with a trivial shade, a stroke length value varies, so it is difficult to use the stroke orientation feature as a robust feature, and although the method using the size of a Kirsch mask extracts and uses 8-direction or 4-direction edge information, since the method using the size of a Kirsch mask utilizes size information obtained by calculating an absolute value rather than orientation information according to the edge information, it is difficult to distinguish an inward orientation of a stroke from an outward orientation of the stroke.
Additionally, when Korean characters are recognized on a grapheme basis, since Korean characters consist of 19 initial sounds, 21 media sounds, and 27 final sounds, the number of possible fonts is 11,172. Accordingly, the grapheme-based Korean character recognition method has a high chance of outputting unused syllables due to combinations of the 19 initial sounds, 21 media sounds, and 27 final sounds rather than recognition of 2,350 completion type Korean fonts. Moreover, it is not easy to separate graphemes, and the grapheme-based Korean character recognition method has a low recognition ratio for fonts including a medial sound, such as “-”, “⊥”, or “”, when strokes of the fonts are thick.
The character recognition method, according to the 6 forms of Hangul consists of 149 first-form Korean fonts, 91 second-form Korean fonts, 109 third-form Korean fonts, 1069 fourth-form Korean fonts, 585 fifth-form Korean fonts, and 347 sixth-form Korean fonts. Thus, the character recognition method, according to the 6 forms of Hangul has a high recognition ratio for the first, second, and third-form Korean fonts having a relatively small number of fonts to be distinguished, whereas the character recognition method according to 6 forms of Hangul has a relatively low recognition ratio for the fourth, fifth, and sixth-form Korean fonts having a large number of fonts to be distinguished.